FIG. 1 depicts one typical rotary valve actuator 100. The rotary valve actuator 100 generally includes a housing 102 and a drive assembly 104. A portion of the housing 102 is removed, thereby exposing the drive assembly 104, which is partly broken away for purposes of description,
The housing 102 includes a generally cylindrical body portion 106 and a pair of opposing cover plates 108a, 108b. The drive assembly 104 includes a diaphragm subassembly 110, shown in cross-section, and a lever subassembly 112. The diaphragm subassembly 110 generally includes an upper housing 114 containing a diaphragm 116, a diaphragm rod 118, and a pair of springs 119. The springs 119 bias the diaphragm 116 into the position depicted. The diaphragm 116 is operably coupled to the diaphragm rod 118 to displace the diaphragm rod 118 against the springs 119 in response to a pressure change in the upper housing 114. The diaphragm rod 118, in turn, actuates the drive assembly 104.
The drive assembly 104 includes a lever 122, a collet 124, and a draw nut 125. The lever 122 includes a body portion 126 and a yoke portion 128. The yoke portion 128 operatively couples the lever 122 to the diaphragm rod 118. The body portion 126 includes a bore 127 defined, at least partly, by a generally cylindrical central portion 126a and a generally frustoconical receiver portion 126b. 
The collet 124 is a generally rod-shaped member disposed within the bore 127 of the body portion 126 of the lever 122 and is adapted for axial displacement relative to the lever 122. The collet 124 includes a plurality of collet fingers 134 and a threaded portion 136. The draw nut 125 threadably engages the threaded portion 136 to secure the collet 124 within the lever 122. The collet fingers 134 have outer surfaces 134a shaped and configured to slidably engage the receiver portion 126b of the lever 122 and inner surfaces 134b shaped and configured to engage a rotary valve shaft 138, which is disposed between the collet fingers 134.
Thus, during assembly, the draw nut 125 is tightened on the threaded portion 136 of the collet 124, thereby drawing the collet 124 to the left relative to the orientation of the actuator 100 depicted in FIG. 1. This causes the collet 124 to slidably displace relative to the body portion 126 of the lever 122. Simultaneously, sliding engagement between the receiver portion 126b and the outer surfaces 134a of the collet fingers 134 causes the collet fingers 134 to displace radially inwardly, thereby wedging between the receiver portion 126b and the valve shaft 138. Continued tightening of the draw nut 125 further displaces the collet 124 to further wedge the collet fingers 134 and secure the assembly 104 to the valve shaft 138.
While the above-described configuration may effectively couple such rotary valve shafts 138 to such actuators 100, they are vulnerable to certain inefficiencies. For example, overdrawing the collet 124 can displace the collet 124 away from the valve shaft 138. This displacement can generate tension on the valve shaft 138. Tension applied to the valve shaft 138 may lead to an offsetting of the valve control element (not shown), which may include, for example, a butterfly valve control element, thereby affecting the quality, life expectancy, or leakage of any seal created thereby. Furthermore, overdrawing the collet 124 can displace the lever 122 toward the valve shaft 138 and into engagement with the housing 102, thereby leading to metal to metal contact, which generates friction and affects the performance of the actuator 100.